“…Soon after the above balloon experiments, many others were successfully performed by Xray astronomy groups spread over many countries (USA, France, Italy, The Netherlands, India, Japan). Most of them were devoted to observations of the Crab, Cygnus region, Sco X-1, Galactic Center region, and, for the first time, hard spectra and time variability were observed (Brini et al 1967;Bleeker et al 1967;Lewin et al 1967;Chodil et al 1968;Haymes et al 1968;Peterson et al 1968;Riegler et al 1968;Overbeck and Tananbaum 1968;Rocchia et al 1969;Glass 1969;Bingham and Clark 1969;Webber and Reinert 1970;Agrawal et al 1971), (Deerenberg and Bleeker 1971;Matsuoka et al 1972).…”